This invention is directed to the selective hydrogenation of dienes to monoolefins particularly of cyclopentadiene to cyclopentene. More specifically, it is directed to a process whereby cyclopentadiene is selectively hydrogenated to cyclopentene through the use of a highly dispersed form of nickel as a catalyst and in which a substantial amount of water and certain surfactants are used as a reaction medium. The surfactants employed are anionic surfactants, cationic surfactants, non-ionic surfactants, amphoteric surfactants, naturally occurring surfactants and oil soluble surfactants.
At the present time, substantial amounts of cyclopentadiene, usually as dicyclopentadiene, are available as a byproduct from the steam cracking of naphtha to produce primarily ethylene. Cyclopentene has been found to be useful as a monomer for the formation of general purpose elastomers by ring opening polymerization of cyclopentene. Therefore, it is desirable to convert a portion of the excess cyclopentadiene available into a more valuable raw material, such as cyclopentene.
The hydrogenation of cyclopentadiene to cyclopentene is not new. For instance, in U.S. Pat. No. 2,360,555 issued Oct. 17, 1944, there is disclosed a selective hydrogenation of one of the two conjugated double bonds of a cyclic diolefin to produce the corresponding cyclic monoolefin which is accomplished by conducting the hydrogenation in the liquid phase in the presence of an active hydrogenation catalyst, under moderate hydrogen pressure such as 2 to 5 atmospheres absolute, and at relatively low temperatures, such as from 0.degree. to 40.degree. C. and even up to 100.degree. C., using substantially less than the stoichiometric amount of hydrogen theoretically required to completely reduce the cyclic diene to the corresponding cyclic monoolefin. The catalyst therein disclosed is a pyrophoric nickel metal catalyst, such as Raney nickel. It is also disclosed that it is desired to conduct the reaction in dilute solution. The dilution may be affected by the addition of any solvent, stable under conditions of the process and which is not a catalyst poison and whose boiling point is such as to render it easily separable from the reaction mixture. Benzene and ethanol, as well as tetralin, dioxane, isooctane, ethyl ether and diisopropyl ether are disclosed as such solvents in such process.
In U.S. Pat. No. 3,819,734, issued July 25, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene by bringing cyclopentadiene into contact with a catalyst consisting essentially of (1) nickel, on a magnesium or zinc oxalate support, (2) a ligand selected from the group consisting of trimethyl phosphine, triethyl phosphine, methyl ethyl propyl phosphine, trimethyl phosphite, triethyl phosphite, tributyl phosphite, triphenyl phosphite, etc., while in the presence of hydrogen, at temperatures above 0.degree. C. and at pressures from 0 to 1000 pounds per square inch gauge. The solvent mentioned therein is ethanol.
In U.S. Pat. No. 3,994,986, issued Nov. 30, 1976, there is disclosed the preparation of cyclopentene from cyclopentadiene by hydrogenating cyclopentene with hydrogen gas at a ratio of 1 to 1.5 moles of hydrogen per mole of cyclopentadiene in the presence of a palladium catalyst on a carrier.
Also, in U.S. Pat. No. 3,857,894, issued Dec. 31, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene in the presence of a palladium catalyst and a small amount of an aqueous solution of zinc salt having a water/zinc ratio of at least 1/1 by weight.
It has been found that in order to have a fairly selective hydrogenation of cyclopentadiene to cyclopentene a reaction medium or diluent should be employed.
Certain advantages are inherent in the use of an aqueous medium and surfactants as the reaction medium or diluent. The presence of water aids greatly in moderating the exothermic nature of the hydrogenation of cyclopentadiene. A particular advantage of the present invention in using a surfactant with the water as the reaction medium is that much better hydrogenation rates can be obtained than when surfactants are not employed. Also, cyclopentadiene and cyclopentene are not soluble in the water-surfactant mixture under the operating conditions employed in the present hydrogenation. Consequently, the process of the present invention provides a very effortless method for removal of the aqueous reaction medium from the product mixture containing the cyclopentadiene because there is formed a two-phase system. Still another advantage which may be obtained is that the cyclopentadiene feedstock which has been formed by the steam cracking of dicyclopentadiene may be employed in an undried condition.
The cyclopentadiene employed in the formation of cyclopentene by hydrogenation is usually obtained by cracking or the depolymerization of dicyclopentadiene. To obtain cyclopentadiene from dicyclopentadiene, dicyclopentadiene is heated to a temperature of 150.degree. C. or higher in a conventional cracking apparatus at atmospheric pressure. The depolymerized material should be used in the hydrogenation process without substantial delay. Cyclopentadiene will redimerize upon storage at ambient temperatures for substantial periods of time.